The invention relates generally to charge transfer devices and, more particularly, to a time-independent charge-coupled device (CCD) charge amplifier.
The basic operation of charge-coupled devices has been explained in detail in the technical and patent literature, but a brief summary of the operation of such devices may facilitate an understanding of the present invention. While the structure of a charge-coupled device will be given in terms of specific semiconductor material types, it will be understood that in general where P-type material is specified, N-type material may be substituted and vice versa.
A typical charge-coupled device may consist of a P-type silicon substrate (in which electrons are normally the minority signal carriers) with a silicon dioxide insulating layer superimposed on its surface. An arrangement of conducting electrodes is deposited on the surface of the insulating layer.
When clock voltages are applied to predetermined groupings of the electrodes, some of the electrons in the vicinity of each electrode, assuming that electrons are initially present (as a result, for example, of injection into the device), will form a discrete packet of charge and move one charge-coupled element, or unit cell, in a predetermined direction for each full clock cycle. The packets of charge move in the predetermined direction as a result of the continuous lateral displacement of the local potential well in which they find themselves. Charge-coupling is thus the collective transfer of all the mobile electric charge stored within a semiconductor storage region to a similar, adjacent storage region by the external manipulation of clock voltages.
The quantity of charge capable of being stored in the mobile packet can vary widely, depending on the applied voltages and on the capacitance of the storage regions. The amount of electric charge in each packet can represent information. Charge-coupled devices have utility in photosensor arrays, delay lines, shift registers, buffer memories, sequential-access memories, fast-access scratchpad memories, refresh memories, and other information storage and transfer mechanisms.
Various types of CCD charge amplifiers are known in the prior art. One type commonly used is the floating gate MOSFET sense amplifier. In the floating gate amplifier a floating gate electrode located above the active CCD channnel senses the magnitude of the signal charge packet. The floating gate in turn is connected to a high impedance amplifier, such as the gate of a MOSFET device. The floating gate CCD charge amplifier is described in Charge Transfer Devices, Sequin and Tompsett, Academic Press Inc., New York, 1975, at pp. 52-58.
Another known type of CCD amplifier, also described in the foregoing publication, is the charge-coupled distributed amplifier, by means of which charge packets are non-destructively sampled at several points along a first CCD line, individually amplified, and combined in a second, larger CCD line. The charge-coupled distributed amplifier illustrated in the foregoing publication utilizes floating gates to detect the charge packets in the first CCD line. A similar-coupled distributed amplifier is described in U.S. Pat. No. 3,806,772. The aforementioned types of CCD charge amplifiers have the disadvantage of being frequency dependent, since they must integrate charge over a fixed period of time.